Rechargeable lithium-ion batteries are now one of the primary power sources for consumer electronics. Such batteries were first produced in high volume by SONY and NEC Moli in the early 1990's. Since then, Li-ion battery technology has been very successful at penetrating high-end consumer electronic markets to replace lead-acid, Ni—Cd and Ni-MH rechargeable batteries. The worldwide annual production of Li-ion rechargeable batteries exceeds 2 billion cells, the majority of which are small size cylindrical and prismatic cells whose capacities are less than 2.8 Ah (ampere hours). Due to their high energy density and long cycle-life compared to other battery technologies, Li-ion batteries are also an attractive technology for larger size, high capacity and high power rechargeable battery markets within the transportation, telecommunication and military applications. Furthermore, economical and environmental constraints have made these batteries the main focus for hybrid electric vehicles (HEV), plug-in hybrid electric vehicles (PHEV), and electric vehicles (EV).
A typical lithium-ion battery consists of a lithium-based transition metal (Mn, Co, Ni) material as the positive electrode, a carbonaceous material (graphite or coke) as the negative electrode, and a non-aqueous electrolyte. The battery also typically has a separator between the positive and negative electrodes, and is typically enclosed in a case. Graphite/LiCoO2, graphite/LiMn2O4, and graphite/LiFePO4 and derivatives of these cell chemistries are the electrochemical energy storage systems of commercial production interest. The electrolyte is typically a solution including a lithium salt, e.g. LiPF6, dissolved in an organic carbonate solvent. After two decades of extensive R&D, this technology appears to have reached a maturity with regard to power and energy density, despite several unsolved weaknesses.
One such weakness is related to energy density. A typical lithium-ion battery can store about 150 watt-hours of electricity per kilogram. For comparison, six kilograms of lead acid battery are required to store the same amount energy. This is a big stride indeed, but there is unlikely to be further improvements in energy density with current materials and designs. Another weakness is with respect to safety. At full charge, carbonaceous anodes are highly reactive because they operate at a potential close to that of metallic lithium, where a film forms. Such films, also known as the solid electrolyte interface (SEI), can be a source of thermal runaway when the electrode is subjected to external or internal heat.
Metallic lithium anodes are not suitable in lithium batteries because they do not form a stable passivation film with conventional electrolytes. Graphite materials are widely used as anode materials in commercial cells, however, the life expectancy of the cells is largely shortened due to irreversibility issues associated with the graphitic unstable solid electrolyte interface. It is also well known that lithium forms alloys with several metals among which silicon and tin. In this case compounds such as Li4.4Si and Li4.4Sn can provide significantly higher capacities. However, using such alloys as anodes with lithium insertion cathode materials leads to volume expansion of the electrodes, which, in turn, leads to cell failure.